1. Field of the Invention
The present invention relates to a hologram erasing method and a hologram erasing apparatus, particularly to the hologram erasing method and the hologram erasing apparatus for selectively erasing a hologram, which has been already recorded, from an optical recording medium.
2. Description of the Related Art
Holographic memory has been receiving attention as a computer file memory of the next generation. The holographic memory has both large capacity derived from a three-dimensional recording region and high speed derived from a two-dimensional batch recording and reproducing method.
In the holographic memory, a plurality of data pages can be recorded by multiplexing the data pages in the same volume, and the data can be read out in a page at a time. The digital data can be recorded and reproduced, not as an analog image but as binary digital data “0,1” digitized in the form of “bright, dark” and recorded and reproduced in the form of the hologram.
In recent years, various proposals on a specific optical system of this digital holographic memory system, S/N ratio or bit error rate evaluations based on volume multiplex recording methods, and two-dimensional coding have been made. Also, studies have been made from a more optical engineering point of view such as influence of aberration of the optical system.
FIG. 7 is a figure showing an angle multiple recording method which is one of the volume multiplex recording methods. In the case of the angle multiple recording method, the hologram is recorded by condensing a digital data page (signal light beam) 2 with a lens 4 and irradiating the same volume within a hologram recording medium 6 with a reference light beam and the signal light beam at the same time. In the case of the multiple recording, the signal light beam is recorded by changing an incident angle of the reference light beam. When reading out the data (reproducing), the objective hologram is accessed by addressing the data with the reference light beam used for the recording, and the data page can be reproduced.
FIGS. 8A and 8B are figures explaining a shift multiple recording method. In this method, a light wave whose wavefront is spacially changed, such as a spherical wave and a speckle pattern, is used as the reference light beam. In the case where such a reference light is used, a Bragg condition for the reproducing can be avoided by only slightly shifting a position of the recording medium by the amount of shift δ from the recording spot (FIG. 8B), and a new hologram can be recorded there. That is to say, the multiple recording of the hologram can be performed in substantially the same volume by slightly shifting the recording medium.
As described above, in the digital holographic storage, high-speed transfer by the two-dimensional batch recording and reproducing method and the increase in the recording capacity by the volume recording can be realized at the same time.
Photopolymer materials, photorefractive materials, photochromic materials such as azopolymer materials, and the like have been actively researched and developed as medium material of the holographic memory. Among these materials, the recording medium using the photorefractive materials or the azopolymer materials can effect erasing the recorded data and recording new data.
Since the recording can be repeatedly performed in these rewritable type recording mediums, in addition to the large-capacity information storage, it is greatly hoped that the rewritable type medium will be utilized as backup memory for hard disk drives or the like.
Conventionally, when the case where the data is erased from the rewritable type recording medium, most generally the erasing is performed by destroying recorded holograms by uniformly irradiating the whole recording region with light. In the recording medium using the photorefractive materials or the azopolymer materials, the holograms can be also erased at once by heating the recording medium and holding the recording medium at a high temperature.
However, in the method in which the recorded information is erased at once, for the recording mediums having large capacities of more than 100 GB, there is a problem in that a long time is required to overwrite new information after all of the information is erased. Therefore, from a practical point of view, it is desirable that only the unnecessary data be erased and the new data be overwritten.
In this case, generally a predetermined hologram is erased by irradiating the predetermined hologram with the reference light beam used for the recording. In this method, a specific apparatus is rendered unnecessary by using the reference light beam for erasing the data, and the hologram can be selectively destroyed by irradiating only the predetermined region with the reference light beam.
Moreover, a method has also been proposed in which a predetermined information is selectively erased by irradiating magneto-optical recording medium, in which medium an interference pattern has been recorded by magnetic reversal of a part irradiated with the light, with the signal light beam and the reference light beam used for the recording, while a magnetic field is applied in a direction which is the reverse of the direction at the time of recording (Japanese Patent Application Laid-Open (JP-A) No. 2001-256654, paragraph Nos. 0078 to 0080).
However, in the method in which the hologram is erased by irradiating the hologram with the reference light beam, there is the problem shown below.
That is to say, in the hologram recording, a change in refractive index or a change in absorption of the medium is induced by irradiating the recording medium with interference fringes generated by the signal light beam and the reference light beam. Regarding light intensity of the interference fringes, the intensity of a strong part reaches several times that of the reference light beam. Accordingly, in order to completely erase the recorded hologram, the reference light beam having the intensity several times as high as that of the recording is necessary in the erasing process. Alternatively, it is necessary to make the reference light beam-irradiating time several times longer than that of the recording.
However, in the case where the hologram is recorded, power of a light source is generally used at full potential to record the hologram at high speed. Accordingly, it is difficult to further increase the laser power of the light source for the reference light beam during the erasing process. On the other hand, when the hologram is irradiated with the reference light beam for a long time during the erasing, high-speed transfer, which is one of merits in the holographic memory, is sacrificed.
In the method in which the information is selectively erased by the signal light beam and the reference light beam used for the recording of the magneto-optical recording medium, it is necessary that the same position as the recording be irradiated with the signal light beam, and there is the problem that position control becomes difficult as an mount of information increases. This method is the data erasing method in the magneto-optical recording in which the interference fringes are used as an assist light beam, and the method can not be applied to the usual hologram recording.